This invention relates generally to the field of anti-tumor therapies. More particularly, this invention provides a method for enhancing the anti-tumor potency and selectivity of certain polyamine analogs. Viewed alternatively, it may be regarded as a drug activated suicide gene therapy.
The polyamine analog N1, N11diethylnorspermine (DENSPM, FIG. 1) is currently undergoing clinical evaluation against various solid tumors (Bergeron et al., 1997, J. Med. Chem. 40: 1475-1494; Porter et al., 1992, Falk Symposium on Polyamines in the Gastrointestinal Tract, Edited by R. H. Dowling, U. R. Folsch, and Chr Loser, Kluwer Academic Publishers, Dordrecht, 1992, pp. 301-322). Under in vitro growth inhibitory conditions, the analog depletes polyamine pools by down-regulating the key polyamine biosynthetic enzymes, ornithine and S-adenosylmethionine decarboxylase, and potently up-regulating the polyamine catabolic enzyme, spermidine-spermine N1-acetyltransferase (SSAT). In many tumor cells, induction of SSAT activity may exceed 1000-fold and this may represent the most potent gene response to any small molecule anticancer agent. Because polyamines, and particularly spermidine, are required for cell growth (Mamont et al., 1978, Biochem. Biophys. Res. Commun. 81:58-66; Kramer et al., 1989, Biochem J. 259:325-331) antiproliferative strategies designed to selectively deplete these molecules from cancer cells have emerged as experimental anticancer therapies (Marton and Pegg, Ann. Rev. Pharmacol. 195, 35:55-91). Induction of SSAT now appears to be a novel means to achieve polyamine pool depletion.
Potent induction of SSAT by DENSPM has been known for some time (Porter et al., supra; Casero et al., 1989, Cancer Res., 49:3829-3833) and several groups have attempted to determine the role of this response in analog mediated growth inhibition. Several lines of evidence support the relationship between SSAT induction and inhibition of cell growth. It is known that both polyamine pool depletion and cell growth inhibition of various cell lines by DENSPM correlates with the extent to which SSAT is induced (Casero et al., 1989, supra; Porter et al., 1991, Cancer Res. 51:3715-3720, 1991; Shappell et al., 1992, Anticancer Res. 12:1083-1090). Analogs that differentially induce SSAT within a single cell line, inhibit cell growth in a manner that is highly correlative with enzyme induction Porter et al., 1991, supra; Kramer et al., 1999, Cancer Res. 59:1278-1286). In other studies, murine embryonic fibroblasts derived from transgenic mice that systemically overexpress SSAT were observed to be more growth sensitive to DENSPM than those from parental animals (Pietilxc3xa4 et al., 1997, J. Biol. Chem. 272:18746-18751; Alhonen et al., 1998, J. Biol. Chem. 273:1964-1969). Finally, DENSPM resistant CHO cells have been shown to be unable to induce high levels of SSAT activity (McCloskey et al., 1999, Proc. Am. Assoc. Cancer Res. 40:302).
While the above observations suggest that induction of SSAT is correlated with inhibition of cell growth, stable over-expression of SSAT does not appear to affect growth rate. It was observed that fibroblasts from transgenic mice that overexpress SSAT, grew at approximately the same rate as parental fibroblasts. A similar observation was recently made with CHO cells transfected with SSAT (McCloskey et al., 1999, J. Biol. Chem. 274, 6174-6182).
Thus, the role of SSAT in growth inhibition at the time of the studies described below was uncertain and despite the known inhibitory effects of DENSPM, in the absence of a clear understanding of the role of SSAT, its anti-tumor effects have not been fully elucidated. Such information could provide valuable strategies for the use and development of polyamine analogs as novel anti-tumor therapies. More particularly, induction of SSAT would provide a useful alternative to the use of polyamine inhibitors and analogs as a novel means to deplete intracellular polyamine pools and inhibit cell growth.
The present invention discloses a method for increasing the anti-tumor potency and selectivity of DENSPM, a polyamine analog. The present method is based on the unexpected observation that increasing the amount of SSAT mRNA, depletes certain polyamine pools, inhibits cell growth and significantly increases the effectiveness of DENSPM. In one embodiment of the invention, SSAT activity is increased by conditional overexpression of SSAT mRNA. In another embodiment, SSAT activity is increased by using a polyamine analog to markedly enhance the expression of the endogenous SSAT gene.
An object of the present invention is to increase the antitumor effectiveness of DENSPM by overexpressing SSAT MRNA prior to treatment with the analog.
Another object of the present invention is to increase the effectiveness of DENSPM by the use of drugs, radiation or factors that enhance transcription of the endogenous SSAT gene.
Another object of the present invention is to provide a conditionally activated gene therapy wherein the induction of SSAT is placed under the control of tissue-specific gene promoter, followed by treatment with DENSPM.